RFR: JDK-8203157: Object equals abstraction for BarrierSetAssembler

Andrew Haley aph at redhat.com
Mon Jun 11 17:11:29 UTC 2018

On 06/11/2018 04:56 PM, Andrew Haley wrote:
> On 06/08/2018 09:17 PM, Roman Kennke wrote:
>> Why is it better? And how would I do that? It sounds like a fairly
>> complex undertaking for a special case. Notice that if the oop doesn't
>> qualify as immediate operand (quite likely for an oop?) it used to be
>> moved into rscratch1 anyway a few lines below.
> Sorry for the slow reply.  I'm looking now.

OK.  The problem is that this is a very bad code smell:

      case T_ARRAY:
        jobject2reg(opr2->as_constant_ptr()->as_jobject(), rscratch1);
        __ cmpoop(reg1, rscratch1);

I can't tell that this is correct.  rscratch1 is used by assembler
macros, and I don't know if some other GC (e.g. ZGC) might need to use
rscratch1 inside cmpoop.  The risk here is obvious.  The Right Thing
to do IMO is to generate a scratch register for pointer comparisons.

Unless, I guess, we know that cmpoop never ever needs a scratch
register for any forseeable garbage collector.

Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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